Register SpatialGizmo as a scene type

(cherry picked from commit ce41464fc0b9a20c5c06f77f0437fa2ef1f9226c)

scene/register_scene_types.cpp

@@ -398,6 +398,7 @@ void register_scene_types() {
 	/* REGISTER 3D */
 
 	ObjectTypeDB::register_type<Spatial>();
+	ObjectTypeDB::register_virtual_type<SpatialGizmo>();
 	ObjectTypeDB::register_type<Skeleton>();
 	ObjectTypeDB::register_type<AnimationPlayer>();
 	ObjectTypeDB::register_type<Tween>();